Starting in the 1980s wildlife officials introduced 72 of the rare rhinos to a protected valley about 200 miles (320 kilometers) southwest of Nepal's capital city of Kathmandu as part of a conservation program.

"We have records showing 23 rhinos had died due to poaching or other causes. The rest are missing," Laxmi Prasad Manandhar, a senior official at Nepal's Department of National Park and Wildlife Conservation, told the Reuters news service.

"Where did they go? I have no answer. It is a mystery," Manandhar said.

The species is also found in the wild in the northeast Indian state of Assam.

In Nepal, army units stationed inside the national parks once effectively deterred poachers and helped the country's rhinos rally from about 60 animals in the mid-20th century to more than 500 in 2000.